As is generally known, in recent years there have been proposed and developed various variable valve actuation devices, in which a working angle of an engine valve (an intake valve and/or an exhaust valve) can be variably controlled depending on an engine operating condition, in order to ensure improved fuel economy and stable driveability (improved operational stability of the engine or stable engine speeds) during low-speed and low-load operation and also to ensure a sufficient engine power output caused by an enhanced intake-air charging efficiency during high-speed and high-load operation. One such variable valve actuation device has been disclosed in Japanese Patent Provisional Publication No. 11-264307 (hereinafter is referred to as “JP11-264307”), corresponding to U.S. Pat. No. 6,041,746, issued on Mar. 28, 2000 and assigned to the assignee of the present invention. The variable valve actuation device disclosed in JP11-264307, often called “continuous variable valve event and lift control (VEL) system”, is comprised of a drive cam integrally connected to an outer periphery of a drive shaft driven by an engine crankshaft, a multinodular-link motion transmission mechanism having a rocker arm and a link member for converting a torque (rotary motion) of the drive cam into oscillating motion, a rockable cam in sliding-contact with an upper face of an intake-valve lifter for transferring an input motion from the motion transmission mechanism and for actuating the intake valve, a substantially horizontally-arranged support arm whose basal end is rotatably supported by the drive shaft and whose tip is rotatably linked to a fulcrum of oscillating motion of the rocker arm of the motion transmission mechanism, and a drive mechanism provided for producing upward or downward rotary motion of the support arm. Also provided is a control means for controlling clockwise/anticlockwise rotary motion of the drive mechanism.
The position of oscillating motion of the rockable cam with respect to the upper face of the valve lifter varies via the rocker arm and the link member by a change in the angular position (clockwise or anticlockwise rotary motion) of the support arm by means of the drive mechanism. Thus, a working angle (a valve open period or a lifted period) of the intake valve can be variably controlled. Additionally, the VEL system disclosed in JP11-264307 is configured such that a phase of the intake valve at its peak valve lift (a maximum valve lift) during the valve open period shifts in a phase-retard direction, as the working angle increases. Therefore, it is possible to greatly change intake valve closure timing, often abbreviated to “IVC”, thereby ensuring the enhanced engine performance.